One mole, each of the two gases $\mathrm{X}$ and $\mathrm{Y}$, are stored separately in two cylinders at $25°\mathrm{C}$ at pressures $1 \mathrm{atm}$ and $2 \mathrm{atm}$, respectively. The difference in the compressibilities of the two gases, $\left({\mathrm{k}}_{\mathrm{x}}-{\mathrm{k}}_{\mathrm{y}}\right)$, is: